1. Field of the Invention
This invention relates to an electroluminescent device.
2. Description of the Prior Art
An electroluminescent device (hereinafter referred to as "EL device") comprises an electroluminescent layer or an active layer (hereinafter referred to as "active layer") sandwiched between a pair of dielectric films, the three layers being further sandwiched between a pair of electrodes at least one of which is transparent. The active layer is caused to emit light by applying AC voltage thereto. To drive this device, a high voltage is required.
The active layer of an EL device is made of ZnS with Mn added to act as luminescent centers and is formed by vacuum evaporation in a thickness of 500 nm to 700 nm. As for the dielectric films, they are made of a material such as Y.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 and are formed by sputtering or the like in a thickness of 400 nm to 800 nm. The transparent electrode is made of indium tin oxide and is formed on a glass substrate, whereas the oppositely placed electrode is made of aluminum. A high voltage of AC 200V to 400V is applied to the active layer through the dielectric films to emit luminescent light.
In view of the above, it is desirable that the dielectric films exhibit a high dielectric constant and a high dielectric breakdown strength. A conventional dielectric film (Japanese Patent Laid-Open Publication No. 52-129296) is composed of a pair of layers deposited together: the material of one of the layers exhibits a high dielectric constant (which means it generally exhibits a high breakdown field strength) and the material of the other layer exhibits a high breakdown field strength (which means it generally exhibits a low dielectric constant). Another conventional dielectric film (Japanese Patent Laid-Open Publication No. 59-224098) is prepared by evenly mixing these two materials, having opposite properties, with each other before forming them into layers.
The problem with conventional film structures is that no consideration is given in them to structurally utilizing the difference in dielectric breakdown characteristics between the above-mentioned materials, which means they are inadequate from the viewpoint of enhancing the dielectric breakdown field strength of an EL device.